Medical electrodes typically include the following layers: (a) a pliable plastic foam pad which is used as the mechanical supporting platform for the other electrode components, (b) an electrically conductive element, for example a thin sheet of tin, silver-silver chloride, or other conductive material, and (c) an electrically conductive electrolyte layer, such as a sponge loaded with wet electrolyte or a self-contained hydrogel electrolyte layer. Electrodes may include other ancillary components such as adhesive layers and protective coverings. The layering sequence of the layers described above would normally be (a),(b),(c), with (c) being utilized closest to the patient and in contact with an electrically conductive organ such as the skin. In normal usage for ECG monitoring or delivery of pacing or defibrillation therapy, these multi-function electrodes would be used in pairs with one being placed on the front of the patient's chest near the apex of the heart and the second electrode being placed either on the skin adjacent the patient's sternum or on the patient's back.
Such medical electrodes are commonly used with electrical instruments to monitor a bioelectrical signal of a patient, e.g., an ECG waveform. Medical electrodes are also typically used to deliver a therapeutic electrical stimulus to the patient, for example a cardiac pacing or defibrillation stimulus.
A typical electrode will have a conductive element made out of metal and/or metal salts plus an electrolyte to couple the conductive element with the patient's skin. During both the acquisition of bioelectrical signals from a patient or the delivery of therapeutic pulses to a patient an electrochemical reaction occurs at the interface of the conductive element and the electrolyte. The exact nature of this electrochemical reaction is dependent on the materials used in the conductive element, how they react with the chemicals in the electrolyte, and the magnitude of the electrical pulse. One such electrochemical reaction, commonly known as polarization, is common to all currently used material combinations for conductive elements and electrolytes when exposed to an electrical pulse. Polarization occurs at the interface of the conductive element and the electrolyte, and its magnitude is largely dependent on the magnitude of the electrical pulse to which it is exposed. As a result, it has not previously been practical to use the conductive elements of a multi-function electrode to acquire an ECG signal immediately following the delivery of a pacing or defibrillation pulse, because the polarization potential (or offset voltage) immediately after a pulse can be on the order of 500-1000 millivolts and the ECG signal to be acquired from the patient typically has a magnitude of a few millivolts.
Therefore, separate ECG electrodes, well spaced away from the pacing or defibrillation electrodes, have historically been required in order to “see” the patient's ECG immediately following a pacing or defibrillation pulse. (Way U.S. Pat. Nos. 4,955,381 and 5,080,099 and Stratbucker U.S. Pat. No. 6,532,379).